1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is driven by a so-called in-plane switching system.
2. Description of the Related Art
In the liquid crystal display devices, a gap formed between a TFT substrate on which pixel electrodes and thin film transistors (TFTs) are formed, and a counter substrate on which color filters, etc., are formed is filled with a liquid crystal, and molecules of the liquid crystal are driven and controlled by an electric field to form an image. Among the liquid crystal display devices, liquid crystal display devices driven by a system called in-plane switching system (IPS system) are frequently used.
The IPS system is a liquid crystal drive system in which liquid crystal molecules are oriented horizontally to a panel surface, and an electric field (lateral electric field) parallel to the panel surface is applied to rotate the liquid crystal molecules in a plane horizontal to the panel surface. In the liquid crystal display device of the IPS system, a common electrode is also formed on a first substrate side formed with video signal lines (drain lines), scanning signal lines (gate lines), thin film transistors, and pixel electrodes, etc. A liquid crystal layer is driven by an electric field in an in-plane direction of the first substrate, which is generated by a difference between voltages applied to the pixel electrodes and the common electrode. In the liquid crystal display device of the IPS system thus configured, for example, linear pixel electrodes overlap with an upper layer of the planar common electrode formed of a transparent conductive film via an insulating film.
Among the IPS systems, in recent years, the following system has been frequently used for the purpose of further increasing the aperture ratio of the liquid crystal display device. In the system, wall structures are so formed as to bridge between the adjacent pixels of the liquid crystal display device, the pixel electrodes are formed on a side wall of the wall structures. Further, the common electrode and a counter electrode are formed on the TFT substrate and the counter substrate which face each other, respectively, and an electric field parallel to the substrate surface is generated to drive the liquid crystal layer.
Also, a gap formed between the TFT substrate and the counter substrate in the liquid crystal display device is as very small as several microns, and it is extremely important to appropriately set the gap between the TFT substrate and the counter substrate for the purpose of controlling light transmission by the liquid crystal. Under the circumstances, there has been proposed that the above wall structures also function as spacers for holding the gap between the TFT substrate and the counter substrate.
Incidentally, in manufacturing the liquid crystal display device, there is a need to pour liquid crystal between the substrates, and seal the liquid crystal therebetween. As a liquid crystal filling method frequently used in recent years, there is a method called one drop fill method (ODF system) in which a required amount of liquid crystal first falls in drops on one substrate, and thereafter is sealed in cooperation with the other substrate to perform filling of the liquid crystal therebetween.
The ODF system is advantageous in that a large-sized manufacturing facility is not required, a time required for manufacturing can be reduced, and the mass production of liquid crystal is facilitated, as compared with the related art liquid crystal injection method. However, a very high precision is required for retaining droplets of liquid crystal, and the above gap between the substrates.
When the ODF system is applied to the liquid crystal display device allowing the wall structures to provide the spacer function as described above, there is a risk that a low-temperature shock bubble may be generated. The low-temperature shock bubbles represent bubbles generated under a low-temperature environment particularly at about −20° C. among so-called vacuum bubbles generated in the case where a negative pressure is generated in the liquid crystal layer, and gas components such as nitrogen which have blended into the liquid crystal layer is eluted therefrom, when a shock caused by an external force is applied to the liquid crystal panel or the like into which the liquid crystal has been poured.
The low-temperature shock bubbles are difficult to again solve, and hardly dissolve, and therefore contribute heavily to the generation of display unevenness or the like. The low-temperature shock bubbles are easily generated in contact portions of the substrates and the spacers, and it is experimentally confirmed that a performance for suppressing the generation of the low-temperature shock bubbles is inversely proportional to contact areas of the spacers and the substrates.
The reason why the generation of the low-temperature shock bubbles is problematic in the liquid crystal display device of the IPS system employing the wall structures is that since a height of the wall structures is kept constant, and the wall structures are formed on all of long sides of pixels, only so-called main spacers are arrayed with a high density, and there is no space in which sub-spacers slightly lower in height than the main spacers are arranged. That is, this is because the contact areas between the substrates and the spacers are increased as compared with a case in which the sub-spacers are arranged.
Also, in manufacturing the liquid crystal display device of the above ODF system where the wall structures also function as the spacers, the wall structures are subject directly to a pressure generated when the substrates are bonded together. This leads to a risk that ITO of electrodes, an interlayer insulating film, or the wall structures per se may be damaged.
JP 2005-157224 A discloses a technique in which the wall structures and supports are arranged between the substrates to regulate a thickness of the liquid crystal layer in the liquid crystal display device of a VA system. Also, JP 2009-145865 A and JP 2010-210866 A disclose a technique in which the thickness between the substrates is retained with the use of the spacers. However, in all of those publications, it is insufficient to suppress the low-temperature shock bubbles of the IPS system, and prevent the wall structures from being damaged.